Merge remote-tracking branch 'quintela/migration.next' into staging

# By Paolo Bonzini
# Via Juan Quintela
* quintela/migration.next:
  migration: simplify writev vs. non-writev logic
  migration: drop is_write complications
  migration: use a single I/O operation when writev_buffer is not defined
  migration: set f->is_write and flush in add_to_iovec

Message-id: 1365512961-15623-1-git-send-email-quintela@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>